This invention relates to borehole amplitude, frequency, and phase measurement systems employing electromagnetic energy propagation in the vicinity of a well borehole. More particularly, the invention relates to a system for generating a desired precision electromagnetic signal in the megahertz frequency range for use in measuring the properties of resistivity and conductivity of rocks in the vicinity of a well borehole.
It is known that the resistivity or conductivity of rocks in the vicinity of a well borehole may be determined by measuring the attenuation of amplitude and the phase shift induced in a propagated electromagnetic wave signal in the megahertz frequency range from 1 MHz to 15 MHz, and particularly in the range of 1 MHz to 5 MHz. The phase measurement is usually performed between two or more spaced receivers spaced apart longitudinally from a transmitter, all of which are carried internally to a sonde or well logging instrument sized for passage through a well borehole. The transmitted signal and the received signals are usually propagated and detected by coil arrays which may be placed circumferentially about the longitudinal axis of such a sonde.
A major problem present in making phase shift measurements relates to the stability and spectral purity of the generator of the electromagnetic energy and the receiver systems used to detect the received signals. The introduction of phase noise into any parts of such systems can lead to significant errors in the measurement of the physical parameters of the rocks to be measured. Such systems for generating and receiving electromagnetic wave energy in the megahertz frequency ranges of interest may generally employ frequency translation by heterodyne mixing, which may also be employed in high frequency communications systems. Since accurate phase shift measurements can usually be performed more accurately at lower frequencies, the heterodyne mixing method can be used to translate the propagated megahertz range electromagnetic wave signal to a much lower intermediate frequency for phase measurement.
Such heterodyne mixing systems generally require a source of two frequency signals, one for the propagated signal or "stimulus" and one for the local oscillator frequency. These frequencies are the RF and LO frequencies. These signals must have a very precise, fixed frequency and phase relationship to assure accurate measurements.
In the system of the present invention, a very precise crystal controlled master oscillator (MO) drives a binary counter, the contents of which are used to address a high speed, high capacity read only memory (ROM). The output from the ROM is supplied to a high speed digital to analog converter (DAC) which supplies a desired radio frequency (RF) output signal to drive transmitter circuitry. A local oscillator signal can be derived from a signal output from the binary counter prior to its input to the ROM. The contents of the ROM can comprise, for example, eight bit digital representations of amplitude samples of a pure sine wave of the desired output frequency for a large number of cycles.